The present invention relates to a device for the air supply of a fuel cell having an expander and a compressor that is at least partially driven thereby in which hot exhaust gases of combustion at least partially flow through the expander.
A device of this type for the air supply of a fuel cell system is known from German Patent DE 197 55 116 C1. In this case, the fuel cell is supplied via a compressor, which is coupled to an expander and an electric motor. The expander in this context at least occasionally supplies a portion of the drive energy for the compressor depending on the operating state. The expander itself is driven by the exhaust gases of the fuel cell, which are after-burned as fuel is added in a catalytic burner.
The disadvantage of a structure of this type lies in the energy requirements for the combustion as well as the structural requirements for the expander. Starting at an exhaust gas temperature level of approximately 80° C. for the fuel cell, which is configured as a PEM fuel cell, a suitably high temperature level must be reached before entering the expander in order for it to be operated effectively. Therefore, in practically all operating states, heating with fuel in the area of the catalytic burner is necessary. Moreover, the achievable temperature is consequently limited by the catalytic burner itself because its catalysts do not enable any temperatures as high as one might desire. Yet, in order to reach as high an energy yield as possible under these preconditions and produce a temperature level after the expander that can be directly emitted into the environment as is without having to accept large energy losses, an accordingly more complex and high-quality expander is necessary. However, this makes the device for air supply disadvantageously very expensive, very difficult, and correspondingly susceptible to any kind of disturbance.
On this basis, an object of the present invention is to provide a device for air supply of a fuel cell of the aforementioned type that avoids the aforementioned disadvantages and which enables a structure that is simple as well as energy-optimized and highly variable with regard to output.
According to the invention, this object is achieved by having the hot gases, after flowing through the expander, emit at least part of the thermal residual energy remaining in them to at least one of the fuel flows supplied for combustion.
The emission of residual thermal energy from the exhaust gases after the expander to at least one of the gases flowing for combustion, which can be, for example, the anode and cathode exhaust gases coming out of the fuel cell, enables a distinct pre-heating of these educts for combustion. Thus, a clearly higher temperature level can be achieved with clearly less fuel consumption than is the case for the prior art.
The fact that the residual energy from the exhaust gases after the expander is expediently used also allows an operation of the expander at temperatures that are better to use in terms of energy than if a temperature approaching the ambient temperature would have to be present after the expander. Thus, the expander can be produced in an accordingly simpler, easier and smaller type of construction than in the prior art and yet have a higher variability with regard to the output that it produces.
According to a very advantageous further development of the invention, the device is configured in such a manner that the exhaust gases after the expander flow through a heat exchanger through which air for the combustion also flows.
This construction is very favorable in that it is especially expedient in terms of energy to preheat the air for combustion because, due to the generally high proportion of inert substances such as nitrogen, it generally has a clearly greater flow volume than fuel and has a clearly higher thermal capacity. This thermal capacity becomes especially high with the previously addressed anode and cathode exhaust gases from the fuel cell because they also still contain water. The fuel, which is supplied at a clearly lower flow volume, will not reach thermal capacities of this kind, so the preheating of the air makes more sense in terms of energy.
In an especially favorable embodiment of the device of the present invention, it can also be provided that, at least during occasional phases of the operation of the device, the exhaust gases, after they have emitted at least a portion of the residual energy contained therein after the expander to the at least one of the fuel flows supplied for combustion, emit additional thermal residual energy remaining in them to a cooling medium for the fuel cells.
In this way, the residual energy that is still contained in the exhaust gases after the preheating of the educts or the air for the combustion can also be used. This especially makes sense in the case of a cold start because in this case, by heating the cooling medium, the fuel cell itself is heated through which this cooling medium also flows. Moreover, in the case of a cold start, higher outputs of the expander may make sense, for example, for the air supply of burners for the preheating of components of a gas generation system or similar system. Thus, there will frequently be a higher temperature level of the exhaust gases anyway, so even the remaining residual energy in the exhaust gases after the preheating of the educts is higher. This can then be very effectively used for preheating the fuel cell and in some cases other components located in the cooling circuit via their cooling medium.
An especially advantageous use of the device of the present invention or one of its possible embodiments provides for installing the device for air supply in a mobile fuel cell system.
For mobile systems of this type, which can be a drive unit for a land, water or air vehicle, such as an auxiliary power unit (APU)—as a function of or independently of a vehicle of this type—a high variability of the output and a lightweight, cost-effective and robust design, as is achieved by the present invention, is especially favorable.